The present disclosure relates to the field of glass-ceramics, and more particularly to the field of transparent dark-colored glass-ceramics that contain a solid solution of β-quartz as the main crystalline phase. Such glass-ceramics can be used as cooking top plates, which cover heating elements such as halogen or radiant hearth type heating elements. Such top plates are generally from 3 to 5 mm thick.
The manufacture of articles made of n-quartz glass-ceramic comprises three main successive steps. A first step includes melting a mineral glass or a mixture of mineral raw materials, which is a precursor of such a glass, and is generally performed between 1550 and 1750° C., followed by refining the molten glass obtained. A second step involves cooling and forming the molten glass obtained. A third step involves crystallization or ceramming of the cooled formed glass, which usually comprises a suitable heat treatment (including steps of nucleation and growth of crystals).
As regards the first step, it may be advantageous to adapt the infrared transmission of the glass to the mode of melting. In a context of using combustion furnaces (with or without supply of electricity), it is thus preferred to have a glass that has a high transmission (in the infrared range) to increase the melting efficiency. As regards the refining operation, suitable refining agents include As2O3, Sb2O3, SnO2, CeO2, and sulfate or fluoro compounds, including mixtures thereof. As2O3 and Sb2O3, more particularly As2O3, have been used extensively. The others, which are less toxic, have more recently been proposed as alternatives to As2O3 and Sb2O3.
The specifications for glass-ceramic cooking top plates, and thus for the glass-ceramic of which they are made, can be particularly stringent. Besides mechanical properties (breaking strength, resistance to heat shocks, etc.) and properties of chemical resistance to acids and bases, which are compatible with their use, such top plates can have specific optical properties. Such optical properties may include (i) a low capacity to transmit visible light, such that the user can not, or can only with difficulty, distinguish the underlying heating elements when they are not in use; (ii) the ability to see, firstly, said heating elements when they are in use, without, however, dazzling the user (so as to reduce the risks of burns on contact with the hot top plate) and, secondly, displays; and (iii) good energy transmission properties, in particular of the infrared radiation produced by the heating elements (so as to enable the foods to be heated in the shortest possible time).
Current cooking top plates, which are dark-colored, are colored with vanadium oxide (V2O5). The vanadium oxide can be added to the raw materials of the glass that is the precursor of the glass-ceramic before performing the melting. It gives the glass-ceramic obtained after ceramming a very deep brown-red shade, associated with the reduction of the vanadium (vanadium having a valency of +5 to vanadium having a valency of +3 and/or +4).
These glass-ceramics colored with vanadium oxide have the optical properties recalled above and, in particular, they allow wavelengths in the red range (above 600 nm) to pass through, such that heating elements brought to high temperature and displays made using electroluminescent diodes that emit in the red range are visible through cooking top plates made of these glass-ceramics. Such top plates are highly absorbent in the wavelength range 450-480 nm corresponding to blue light. Thus, they transmit little or no color emitted by blue electroluminescent diodes.
It has recently been found that there is a need to be able also to see blue-colored displays through such glass-ceramics top plates. To satisfy this need, top plates are thus proposed, which, besides the properties, more particularly the optical properties, recited above, must have a non-zero capacity to transmit wavelengths of the visible range between 450 and 480 nm (limits inclusive), corresponding to blue light.
U.S. Pat. No. 5,212,122 describes colored transparent glass-ceramics whose transmission in the infrared range may be adjusted by the action of dye(s) chosen from MnO2, Fe2O3, CoO, NiO, V2O5, Cr2O3 and mixtures thereof. Vanadium oxide is not necessarily present in said glass-ceramics. The effect on the transmission in the infrared range of the dyes CoO, NiO and V2O5 is not differentiated. The technical problem of transmission in the blue range is not addressed.
European patent application EP 1 313 675 describes transparent glass-ceramics containing a solid solution of β-quartz as a main crystal phase. Said glass-ceramics do not contain either As or Sb. Rather, the chemical refining of the precursor glass is performed with an alternative refining agent chosen from SnO2, CeO2, and sulfate or fluoro compounds. The chemical refining is performed at high temperature (at a temperature above 1700° C. and especially above 1975° C.) to give high-quality results, and they are colored with V2O5 combined with at least one reducing agent. Said glass-ceramics have an optical transmission at 1600 nm of greater than 65% for a thickness of 4 mm. The technical problem of transmission in the blue range is not addressed in said document.
European patent application EP 1 465 460 mentions glass-ceramic cooking top plates whose light transmission Y, integrated over the entire visible spectrum, measured with the illuminant C, is from 2.5 to 15 at a thickness of 3 mm. Such high transmission inevitably leads to the possibility of seeing the heating elements through the cooking top plate when it is not in use. Moreover, said patent application recommends the use, in order to obtain the desired results, of an oxidized precursor glass, which has little coloration since the vanadium present is predominantly in the oxidized state (V5+). During ceramming of said glass, said vanadium is reduced, especially by arsenic and/or iron, resulting in the dark color of the final glass-ceramic. It is, however, more than likely that said vanadium is not entirely reduced during the ceramming, and that it continues to be reduced in the course of using the cooking top plate as a result of the high temperatures to which said cooking top plate is subjected. This results in a phenomenon of aging of the cooking top plate. It gradually darkens over time. In any case, EP 1 465 460, in reference to the technical problem of the transmission of blue, green, yellow, red, in fact of the transmission of all colors (see the high Y values indicated), develops an approach essentially based on reduction of the vanadium.
With reference to the specific problem of the selective transmission of blue, another approach includes coloration via the combined action of dyes.